The invention relates generally to computer printers and more particularly to high-speed laser printers.
In a large main frame computer, e.g. a CRAY-2 or CRAY-XMP supercomputer, millions of calculations per second are performed. The large amounts of data produced must be output in usable form. Thus, the printer becomes an important component of the computer system. Graphics data is often more difficult to print than text because of the much greater volume of data needed to form the graphic image. Printing of graphics can be a slow and expensive procedure.
In a laser printer a laser beam modulated by an acousto-optical modulator is scanned by means of scanning optics across a photoconductive surface on a rotating drum to produce an image on the drum. The image on the drum is then transferred to paper by a xerographic process. A laser printer is a serial writing device, i.e. one bit of data at a time. This serial data is sent to the acousto-optic modulator which modulates the laser beam. A serializer is used to read out a bit image from memory in parallel mode, e.g. 16 bits at a time, and then serialize the image bits for transmission one bit at a time to the acousto-optic modulator.
The IBM model 3800-3 laser printer is a very high-speed laser printer which has an absolute speed of 3.7 pages per second. Text can be printed at about full printer speed, 3.6 pages per second. However, printing graphics is much slower, at best 7-10 seconds per page because the standard I/O channel is too slow for the voluminous graphics data required to print a page. Therefore it is highly desirable to speed up printing of graphics to approach full printer speed, particularly in a computing environment where large amounts of graphics data are produced.
A number of other printing systems have been developed. U.S. Pat. No. 4,672,459, to Kudo shows a communication terminal apparatus with data format conversion capability for a facsimile machine. U.S. Pat. No. 4,677,571, to Riseman discloses a printing system which converts data to index numbers for a matrix of points representing certain shapes as well as gray scale. U.S. Pat. No. 4,367,533, to Wiener shows an image bit structure and method for increasing size of characters. U.S. Pat. No. 4,635,212, to Hatazawa rotates a print pattern. U.S. Pat. No. 4,527,918 to Yamamoto shows a dual head printer, one for characters and one for plotting graphics. Thus, the prior art does not deal with the problem of reduced printer speed for graphics printing and methods for increasing printer speed to its inherent speed.